7,500 a year might not be a huge chunk of a captains salary, but realistically you won't be earning money of any form pretty much until your late 20's I would guess and that is being very optimistic, becoming an airline pilot is very very competitive and you will be doing mail runs etc at best for the first few years as a job. The pay is basically like earning dirt and trying to use it for cash. Your first years working as a pilot are doing anything and everything to get your hours up, I was told as a general idea of what to aim for you won't even be looked at for a job as a commercial pilot with an airline with anything under about 2000 hours.
That being said though, go for it for sure, like most people have said, it is your dream job and on top of that you will get to travel a bit around which is absolutely awesome. You are also only 17 and almost have your PPL's already which is a good effort and obviously shows you are willing to put in the time and dedication that is needed for it, and if you are a smart enough person that you won't get snowed under with everything that comes from being a pilot your definately on the right track.
I would also like to put in a +1 for the people who have said to go the RAF option, that way you get paid to train and it is by far the best training for a pilot, far better option than doing it the commercial way, plus you meet some awesome people through the defence force as well. If you haven't looked into this option, do it before you make a decision with the £75,000 way.